Hydrothermal aqueous mineralizer for growing optical-quality single crystals

ABSTRACT

A hydrothermal process is disclosed for growing a crystal at MTiOXO 4  wherein M is NH 4 , K, Rb and/or Tl or mixtures thereof with Cs and X is P and/or As, at elevated temperatures using a mineralizer comprising both M +1  and X +5  and an amount of F -0  effective to increase the solubility of MTiOXO 4  in the mineralizer. Mineralizers containing F -1 , M +1  and X +5  are disclosed wherein F -1  is present in an amount effective to increase the solubility of MTiOXO 4  in the mineralizer and to provide a MTiOXO 4  solubility of at least about 1% by weight at the operating temperature and pressure for the hydrothermal production.

This is a division of application Ser. No. 07/843,679, filed Feb. 28,1992, now U.S. Pat. No. 5,264,073.

FIELD OF THE INVENTION

This invention relates to producing optically useful crystals ofMTiOXO₄, wherein M is selected from the group consisting of NH₄, K, Rb,Tl, mixtures thereof and mixtures of Cs therewith, and X is selectedfrom the group consisting of P, As and mixtures thereof, and moreparticularly to hydrothermal processes for producing such crystals.

BACKGROUND OF THE INVENTION

The need for optical quality single crystals of materials exhibitingnonlinear optical properties is well established in the art. Potassiumtitanyl phosphate (i.e. KTP) is particularly useful in nonlinear opticaldevices, as described, for example, in U.S. Pat. No. 3,949,323. Opticalquality crystals having dimensions on the order of one millimeter ormore are particularly useful for many optical applications.

Processes for producing optical quality crystals using aqueous systemsare known generally in the art as hydrothermal processes, and processesfor producing such crystals using nonaqueous molten salt systems areknown generally in the art as flux processes. Hydrothermal processes areconsidered particularly advantageous relative to flux processes forgrowing crystals for certain applications since hydrothermal processesoften produce crystals having better optical damage resistance andrelatively lower ionic conductivity. Typically, hydrothermal processesinvolve growing the crystal in a vessel having a growth region where thecrystal grows and a nutrient region containing nutrient for growing thecrystal, and employ an aqueous mineralizer solution (i.e., amineralizer).

For example, one type of mineralizer used for hydrothermal crystalgrowth of KTP uses a mineralizer containing both potassium andphosphate. A typical commercial production of KTP crystals, described byBelt et al., SPIE Proceedings, 968, 100 (1988), uses a potassiumphosphate mineralizer at a temperature in the range of about 520° C. to560° C. and a pressure in the range of about 1700 to 2000 atmospheres.The relatively high temperature and pressure employed in this processmakes scale-up difficult and expensive. As suggested in Laudise et al.,"Solubility and P-V-T Relations and the Growth of Potassium TitanylPhosphate", Journal of Crystal Growth, 102, pp. 427-433 (1990) the useof more moderate conditions can lead to a problem of the coprecipitationof anatase (TiO₂), making the process less useful. Mineralizers rich inpotassium have been used under conditions of fairly moderate temperature(e.g.,275° to 425° C.) and pressure (e.g., less than 14,000 psi) asdescribed in U.S. Pat. No. 5,066,356. Although U.S. Pat. No. 5,066,356demonstrates that growth temperature and pressure can be reduced whileusing certain concentrated mineralizer solutions, the growth rate ofthat process is somewhat limited due to the relatively low solubility ofKTiOPO₄ in the mineralizer. U.S. Pat. No. 4,305,778 discloses ahydrothermal process for crystal growth which utilizes a stable glasscomposition that minimizes the tendency of the seed crystals to dissolvein the aqueous mineralizer solution before nutrient can migrate to theseed crystals.

Another type of mineralizer used for the hydrothermal growth of KTiOPO₄involves the use of potassium fluoride solutions as described, forexample, in Jia et al., "The Solubility of KTiOPO₄ (KTP) in KF AqueousSolution Under High Temperature and High Pressure", Journal of CrystalGrowth, 79 (1986), pp. 970-973, and in Jia et al., "Hydrothermal Growthof KTP Crystals in the Medium Range of Temperature and Pressure",Journal of Crystal Growth, 99 (1990), pp. 900-904. Jia et al. disclosethat by utilizing KF as a mineralizer relatively lower temperature andpressure can be employed for a hydrothermal KTP crystal growth process,and use of a pressure as low as 1000 Kg/cm² (i.e. 14223 psi) isexemplified. The use of pure KF mineralizer as described by Jia et al.provides a solubility of about 2% under the stated growth conditions,but like the process described in Laudise et al. supra, the highersolubility occurs near the phase stability boundary (with respect totemperature, pressure and mineralizer concentration), so the possibilityof coprecipitation of an undesirable non-KTP phase exists.

A generally recognized family of KTP-type materials has the formulaMTiOXO₄ where M is selected from the group consisting of NH₄, K, Rb, Tl,mixtures thereof and mixtures of Cs therewith, and X is selected fromthe group consisting of P, As and mixtures thereof. Hydrothermalprocesses for producing KTiOPO₄ are well studied. A problem associatedwith extending these studies to hydrothermal production of othercrystals of the family involves the compositional variability of themineralizers most suitable for growing each of the MTiOXO₄ family ofmaterials. These variations make the development of new crystals, suchas KTiOAsO₄, difficult and costly, since a new set of desirable growthparameters must be found for each of the isomorphs and their solidsolutions.

SUMMARY OF THE INVENTION

This invention provides a method of increasing the rate of crystalgrowth in a hydrothermal process for growing a crystal at MTiOXO₄,wherein M is selected from the group consisting of NH₄, K, Rb, Tl,mixtures thereof, and mixtures of Cs therewith and X is selected fromthe group consisting of P and As and mixtures thereof. In general, themethod is used with a hydrothermal process for growing said crystal in agrowth region at elevated temperatures using a mineralizer comprisingboth M⁺¹ and X⁺⁵, and comprises the step of employing a mineralizerfurther comprising F⁻¹ in an amount effective to increase the solubilityof MTiOXO₄ in the mineralizer.

This invention further provides an aqueous mineralizer for thehydrothermal production of crystals of MTiOXO₄, consisting essentiallyof an aqueous composition (solution/mixture) of F⁻¹, M⁺¹ and X⁺⁵ whereinF⁻¹ is present in an amount effective to increase the solubility ofMTiOXO₄ in the mineralizer and to provide a MTiOXO₄ solubility of atleast about 1% by weight at the operating temperature and pressure forthe hydrothermal production. Optical quality crystals of at least 1 mm³can be grown using the mineralizer of this invention in times on theorder of hours, rather than days and weeks.

It is an object of this invention to provide an improved mineralizer foruse in hydrothermal processes to improve crystal growth rates for theMTiOXO₄ family of materials by increasing the solubility of MTiOXO₄ inthe mineralizer.

It is a further object of this invention to provide mineralizerembodiments for the growth of MTiOAsO₄ materials at relatively lowtemperature and pressure.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a ternary diagram which depicts the broad and preferred rangeof aqueous mineralizer composition for the production of MTiOXO₄ at afluoride molality of from about 4 to 12, and from about 400° C. to 600°C. and from about 1000 to 2000 atmospheres pressure.

DETAILED DESCRIPTION

A hydrothermal process is provided in accordance with this inventionwhich employs a concentrated aqueous mineralizer solutions comprisingF⁻¹ in addition to M⁺¹ and X⁺⁵ in a hydrothermal process to achieverelatively rapid, low pressure and/or low temperature growth ofcrystalline materials of the formula MTiOXO₄, wherein M is selected fromthe group consisting of NH₄, K, Rb, Tl, mixtures thereof and mixtures ofCs therewith (preferably K) and X is selected from the group consistingof P and As and mixtures thereof. An aqueous mineralizer solution isalso provided in accordance with this invention which consistsessentially an aqueous composition of F⁻¹, M⁺¹ and X⁺⁵ (where M and Xare defined above). The hydrothermal process for crystal growth atelevated temperature in a growth region, employs a growth mediumcomprising a mineralizer solution containing F⁻¹, M⁺¹ and X⁺⁵ andemploys either a growth region temperature from about 200° C. to about800° C. or a pressure of less than 28,000 psi, or both, duringcrystallization. Typically, growth rates greater than 1 mm/side/week canbe achieved at even the relatively lower temperatures studied in the art(i.e., 200° C. to 600° C.), and at operating pressures significantlylower than 8,000 psi, and preferably about 3,500 psi or less. For thebest combination of effective crystal growth rate and economics ofoperation, the crystal growth pressure for the present invention iscommonly in the range of from 2,000 to 14,000 psi. A typical temperaturerange is from about 250° C. to about 450° C.

The general procedure used for hydrothermal crystal growth is well knownin the art and involves conducting the crystal growth at elevatedtemperature and pressure in a pressure vessel containing a means fornucleating crystal growth and a growth medium comprising a nutrient andan aqueous mineralizer solution. The preferred means for nucleatingcrystal growth are seed crystals which provide nucleation sites. Inaccordance with this invention seed crystals of MTiOXO₄, as definedabove, can be utilized. Typical hydrothermal processes usually use ahigh pressure vessel or container in which single crystal seeds of thedesired product are hung in a relatively supersaturated zone (i.e., agrowth region) and a large quantity of polycrystalline nutrient of thesame material is maintained in an unsaturated zone (i.e. a nutrientregion), all together with an aqueous solution of mineralizer in whichthe nutrient is suitably soluble. The temperatures of the growth regionand the nutrient region are selected in accordance with the slope of thesolubility/temperature correlation for the specific mineralizer used.For solutions having a positive (normal) solubility/temperaturecorrelation, the nutrient region is kept warmer than the growth region,while for solutions having a negative (retrograde)solubility/temperature correlation, the growth region is kept warmerthan the nutrient region. Under steady-state growth conditions, thenutrient dissolves in the hotter (or cooler in the case of a retrogradecorrelation) nutrient zone, is transported to the cooler (or hotter inthe case of a retrograde correlation) growth zone via natural convectioncaused by a density gradient or by forced convection (e.g., a rockermechanism) and is deposited on the seed crystals. Large crystals arethereby produced.

The aqueous solution of mineralizer in combination with a nutrientconsisting of a polycrystalline form of MTiOXO₄ or in combination withsuitable precursors thereof makes up the growth medium. The amount ofnutrient is not critical, provided enough is present to saturate thegrowth medium and provide sufficient material for the desired massincrease on the seed crystal(s). Nutrient surface area is typically 5 to10 times the surface area of the seed crystal(s) so that the rate ofnutrient dissolution does not undesirably limit crystal growth.

The mineralizer should contain both M⁺¹ and X⁺⁵ as defined above and, inaccordance with this invention must contain fluoride to achievedesirable growth rates. For example, materials such as described by Beltet al., "Nonlinear Optic Materials for Second Harmonic Generation(KTP)", Avionics Laboratory, Air Force Wright Aeronautical Laboratories,Air Force Systems Command, Wright-Patterson Air Force Base, Ohio (1984),formed from the combination of MH₂ XO₄ and M₂ HXO₄ are suitable for usealong with MF. It is noted that the F⁻¹ provided for use in thisinvention need not be specifically added as the compound MF, but can beadded using other fluoride sources which do not interfere withcrystallization (e.g., a mixture of HF and MOH) which form MF in situ.The X⁺⁵ is generally present in the mineral or combination as an oxide(e.g., XO₄ ⁻³), and may be provided by adding compounds such as X₂ O₅,and/or MH₂ XO₄. In accordance with this invention it is preferred thatthe mole ratio of M.sup. +1 to X⁺⁵ in the mineralizer solution is fromabout 0.7:1 to about 2.5:1 to avoid formation of material other than thedesired MTiOXO₄, such as anatase (TiO₂). If desired, an oxidizing agent,such as KNO₃ or H₂ O₂, can be present in the growth medium in smallconcentrations to prevent reduction of Ti⁺⁴ and to enhance crystalquality.

In accordance with the practice of this invention the concentration ofF⁻¹ in the aqueous solution of mineralizer is preferably at least about1 molal (as F), more preferably 2 molal or more, and is preferably 12molal or less. The concentration of M⁺¹ in the aqueous solution ofmineralizer is preferably at least 1 molal (as M), more preferably 2molal or more, and is preferably 20 molal or less, more preferably 16molal or less. A concentration of M⁺¹ from about 2 to 16 molal isconsidered particularly suitable for many applications. It is recognizedthat concentrations of M⁺¹ as high as 20 molal may not be achieved atroom temperature, but can be achieved at the higher temperatures ofoperation. Consequently, solid material might be added with the aqueoussolution of mineralizer, which would dissolve upon heating to provide anaqueous solution of mineralizer of the desired concentration. Theconcentration of X⁺⁵ in the aqueous solution of mineralizer ispreferably at least about 0.2 molal (as X), more preferably 1 molal ormore, and is preferably 12 molal or less, more preferably 8 molal orless. The invention is considered particularly useful for growingcrystals where X is As.

An aqueous mineralizer is provided in accordance with this invention forhydrothermal production of crystals of MTiOXO₄ which consistsessentially of an aqueous composition containing F⁻¹, M⁺¹ and X⁺⁵wherein F⁻ is present in an amount effective to increase the solubilityof MTiOXO₄ in the mineralizer and to provide MTiOXO₄ solubility of atleast about 1 percent by weight of the operating temperature andpressure of a hydrothermal crystal growth process.

While pH of the aqueous mineralizer of the present invention is notparticularly critical to the crystal growth process per se, the pH canbe important in the construction of pressure vessels for the process.For example, low pH (about 7 or less) and low operating temperature andpressure (e.g., 200° C. and 400 atmospheres) allows use of metal vesselswith liners other than noble metals (e.g., Teflon®).

Certain proportions of M⁺¹, F⁻¹ and X⁺⁵ are considered particularlysuitable for mineralizer compositions. Mineralizer systems of thisinvention may be represented on a ternary diagram, in terms of therelative mole percent of MF, MOH and X₂ O₅. FIG. 1, for examplerepresents a ternary diagram for mineralizer used for growing MTiOXO₄,wherein mole percentages in terms of MF, MOH and X₂ O₅ are shownrelative to each other (water of aqueous mineralizer is not included).The corners of the ternary diagram labeled MF, MOH and X₂ O₅ thusrepresent 100 relative mole percent total of MF, MOH and X₂ O₅respectively; and each point within the diagram represents 100 molepercent total of MF, MOH and X₂ O₅. Preferably for the growth of MTiOXO₄at about 4 to 12 molal F⁻¹ and at about 400° to 600° C. and about 1000to 2000 atmospheres pressure, the amounts of M, F and X are selected toprovide a mineralizer compound falling within the polygon which isdefined by III III IV V VI I; and more preferably the amounts of M, Fand X are selected to provide a mineralizer composition falling withinthe polygon which is defined by A B C D E F A. The diagram is especiallyuseful for defining preferred ranges where M is K and X is P or As.Similar ranges apply to other MTiOXO₄ isomorphs. At lower temperatures(e.g., less than about 300° C.) and absolute fluoride concentrationsbelow 4 molal, these zones expanded slightly to include the pure KFmineralizer as well.

While the practice of this invention is not bound by any theory orexplanation, the role of fluoride ion in the aqueous mineralizer of thisinvention is believed to be related to the dissolution of MTiOXO₄. Thedissolution can be written as follows:

MTiOXO₄ (solid)→M⁺ +Ti⁺⁴ (complexes)+XO₅ ⁻⁵ (complexes)

In a highly concentrated solution of M⁺¹ and/or X⁺⁵, such as typicallyexists in MOH/X₂ O₅ -type aqueous mineralizer solutions equilibrium isshifted to the left in accordance with the law of mass action, favoringthe precipitation of MTiOXO₄ and lowering its solubility. To obtain highsolubility, one should therefore (1) find ions which readily formwater-soluble complexes with titanium (Ti⁺⁴); and (2) reduce theabsolute concentration of M⁺¹ and/or X⁺⁵. In accordance with thisinvention, fluoride ion complexes effectively with titanium and isbelieved to be primarily responsible for the high solubility of MTiOXO₄in the mineralizers of the present invention. As is evident in FIG. 1,at high temperature (above about 400° C.) and high F⁻¹ concentrations(above about 4 molal), the amount of M and/or X cannot be reduced tozero because of the MTiOXO₄ -phase stability requirements. Accordingly,the addition of M and X stabilizes the MTiOXO₄ phase field in thepresence of fluoride.

A typical hydrothermal process using aqueous mineralizer of thisinvention comprises the steps of (1) providing in a vessel (a) means fornucleating growth of a crystal at MTiOXO₄ in a growth region, (b) agrowth medium comprising nutrient for growing said crystal of MTiOXO₄and an aqueous solution of mineralizer containing a mixture of F, M(i.e., NH₄, K, Rb, Tl, or partial Cs) and X (i.e., As or P) in anutrient region, and (c) means for producing a temperature gradientbetween said growth region and said nutrient region; (2) employing insaid nutrient region a nutrient temperature sufficient to effectsolution of at least a portion of said nutrient; and (3) employing insaid growth region an elevated growth temperature at which the MTiOXO₄has a lower solubility than at said nutrient temperature and a pressurewhereby growth of said crystal proceeds.

Under the mild conditions of temperature and pressure typically employedin the practice of this invention, the vessel chosen can be selectedfrom a wide variety of types and sizes which can withstand thesereaction temperatures and reaction pressures. Pressure vesselconfigurations typically utilize a noble metal container, or noblemetal-clad container although other inert material such as Teflon® canalso be used for lining the container. Platinum, gold and Teflon® arepreferred liner materials for the crystal growth of potassium titanylphosphate and potassium titanyl arsenate. A ladder-like rodconfiguration can be used to hang a number of seed crystals in thegrowth region portion of the lined vessel and a perforated baffle plateis used to separate the part of the container containing the growthregion from the part containing a nutrient region. (The growth regioncan be located either above or below the nutrient region, depending uponthe type of temperature/solubility correlation.) The baffle plate aidsin maintaining the thermal gradient during growth. Nutrient is placed inthe nutrient region, separated from the growth region by the baffle. Anaqueous solution of mineralizer is loaded into the can along with theseed crystals. The can is sealed and typically placed in an autoclavewith a sufficient amount of water to generate a pressure on the outerwall equal to the pressure generated inside the can at the predeterminedmaximum operating temperature. This "pressure balancing" approachprevents the can from collapsing and thus enables proper crystal growth.Typically, if a noble metal liner is used the need for the can iseliminated. Various conventional forced convection techniques such asmechanical rockers or stirrers can also be employed for the crystalgrowth process. Alternatively, a horizontal rather than verticalconfiguration can be utilized.

The percentage of fill is defined as the room temperature atmosphericpressure volume of the solution divided by the free volume of thecontainer, i.e., the container volume less the volume of nutrient, seedcrystals, frame or ladder, and baffle. In the case of a lined vessel,the container's volume is the volume of the vessel. Common practice inthe art has generally been to limit the percentage of fill so thatexcessive pressure is not encountered at the growth temperature, and apreferred fill range from 70 to 85% has typically been used depending onmineralizer concentration. Percentages of fill above about 85% havegenerally been avoided because the resulting pressure is generallyconsidered excessive for assuring vessel integrity. In accordance withthis invention percentages of fill of from about 60% to 95% arepreferably used, with fill percentages of from about 70% to 85% beingmost generally preferred.

The following non-limiting examples further illustrate practice of theinvention.

EXAMPLES

Hydrothermal solubilities of MTiOXO₄ may be studied using the followingstandard solution-quenching method. A mineralizer mixture consisting oftypically 2 to 4 ml of H20 and varying amounts of MF, X₂ O₅ and/or MH₂XO₄, is placed in a 0.375 inch gold or platinum tube along with apre-weighed amount of excess MTiOXO₄ powder. The two ends of the tubeare then welded shut, and the tube is placed in an internally heatedpressure vessel and brought to a specified temperature and pressure. Thesolution is held at this temperature and pressure until equilibriumsolubility is established (typically from 1 to 3 days). The pressurevessel is then quenched quickly (about 600° C./min.) to roomtemperature. Rapid cooling is used to keep all dissolved MTiOXO₄ insolution. Upon cutting-open the tube, the pH of the solution is measuredand the undissolved MTiOXO₄ powder and crystals are recovered andweighed. The MTiOXO₄ weight loss is taken as the hydrothermal solubilityat the temperature and pressure condition used.

EXAMPLE 1

The standard solution-quenching method outlined above was used todetermine the relationship of solubility to pH at from 400° C. to 600°C. and from 1000 atmospheres to 2000 atmospheres. Results for a varietyof aqueous mineralizer compositions in accordance with this inventionfor the production of KTiOAsO₄ are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        pH's of Ouenched Mineralizer Solutions and its                                Associated Solubilities for KTA.sup.1                                         Mineralizer Solutions % Solubility                                                                             pH                                           ______________________________________                                        12M KH.sub.2 AsO.sub.4 /2.4M KOH (Control)                                                          0.24       9                                             8M KF/6.9M KH.sub.2 AsO.sub.4                                                                      7.97       7                                             8M KF/4M KH.sub.2 AsO.sub.4 /2M As.sub.2 O.sub.5                                                   9.59       5                                             4M KF/1.7M KF.sub.2 AsO4/0.9M As.sub.2 O.sub.5                                                     5.07       4                                             4M KF/1.3M As.sub.2 O.sub.5                                                                        5.93       3                                             4M KF/1.7M As.sub.2 O.sub.5                                                                        5.66       2                                             4M KF/2.5M As.sub.2 O.sub.5                                                                        4.50       1                                            ______________________________________                                         .sup.1 KTiOAsO.sub.4                                                     

The Control mineralizer known in the art (i.e., mineralizer containingno KF) has a very low solubility.

EXAMPLE 2

The standard solution-quenching method outlined above was used todetermine the solubility of MTiOXO₄ for a variety of isomorphs ofMTiOXO₄ in the aqueous mineralizer of the present invention. Results areshown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    Hydrothermal Solubility of Various KTP                                        Isomorphs in KF Containing Mineralizer Solutions                                                             %                                                     Mineralizers   Temp./Press.                                                                           Solubility                                                                         Ti.sup.9                                  __________________________________________________________________________    KTP.sup.1                                                                            4M KF/2M P.sub.2 O.sub.5                                                                     400° C./1000 atm                                                                3.96 0.30                                      RTP.sup.2                                                                            4M RbF/2M P.sub.2 O.sub.5                                                                    400° C./1000 atm                                                                5.72 0.40                                      TTP.sup.3                                                                            4M TlF/2M P.sub.2 O.sub.5                                                                    400° C./1000 atm                                                                6.94 0.66                                      KTA.sup.4                                                                            4M KF/2M As.sub.2 O.sub.5                                                                    400° C./1000 atm                                                                4.72 0.33                                      RTA.sup.5                                                                            4M RbF/2M As.sub.2 O.sub.5                                                                   400° C/1000 atm                                                                 5.20 0.34                                      TTA.sup.6 T                                                                          4M TlF/2M As.sub.2 O.sub.5                                                                   400° C./1000 atm                                                                7.40 0.66                                      KTP    4M KF/8M KH.sub.2 PO.sub.4                                                                   600° C./2000 atm                                                                2.38 0.28                                      RTP    4M RbF/8M RbH.sub.2 PO.sub.4                                                                 600° C./2000 atm                                                                2.43 0.28                                      NTP.sup.7                                                                            4M NH.sub.4 F/4M NH.sub.4 H.sub.2 PO.sub.4                                                   200° C./1000 atm                                                                4.48 0.28                                      KTA    4M KF/8M KH.sub.2 AsO.sub.4                                                                  600° C./2000 atm                                                                2.69 0.30                                      RTA    4M RbF/8M RbH.sub.2 AsO.sub.4                                                                600° C./2000 atm                                                                2.68 0.30                                      NTA.sup.8                                                                            4M NH.sub.4 F/4M NH.sub.4 H.sub. 2 AsO.sub.4                                                 200° C./1000 atm                                                                5.92 0.30                                      R.sub.0.5 K.sub.0.5 Ta                                                               4M (K + Rb)F/8M KH.sub.2 AsO.sub.4                                                           600° C./2000 atm                                                                4.42 0.49                                      KTP.sub.0.5 A.sub.0.5                                                                4M KF/8M KH.sub.2 (P + As)O.sub.4                                                            600° C./2000 atm                                                                4.34 0.49                                      Cs.sub.0.67 K.sub.0.33 TP                                                            4M CsF/8M KH.sub.2 PO.sub.4                                                                  600° C./2000 atm                                                                6.84 0.65                                      __________________________________________________________________________     .sup.1 KTiOPO.sub.4                                                           .sup.2 RbTiOPO.sub.4                                                          .sup.3 TlTiOPO.sub.4                                                          .sup.4 KTiOAsO.sub.4                                                          .sup.5 RbTiOAsO.sub.4                                                         .sup.6 TlTiOAsO.sub.4                                                         .sup.7 NH.sub.4 TiOPO.sub.4                                                   .sup.8 NH.sub.4 TiOAsO.sub.4                                                  .sup.9 molal Ti concentration in the mineralizer                         

EXAMPLE 3

The standard solution-quenching method outlined above was used todetermine the solubility of KTiOPO₄ and KTiOAsO₄ at various operatingtemperatures and pressures. Results are shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________    Temperature and Pressure Dependence of Hydrothermal                           Solubility in KF Containing Mineralizer Solutions                                                         % Solu-                                           Mineralizers       Temp./Press.                                                                           bility                                                                             Difference.sup.3                             __________________________________________________________________________    KTP.sup.1                                                                          4M KF/8M KH.sub.2 PO.sub.4                                                                  400° C./1000 atm                                                                1.34                                                   4M KF/8M KH.sub.2 PO.sub.4                                                                  600° C./2000 atm                                                                2.38 1.04                                              4M KF/2M P.sub.2 O.sub.5                                                                    400° C./1000 atm                                                                3.96                                                   4M KF/2M P.sub.2 O.sub.5                                                                    600° C./2000 atm                                                                3.20 -0.76                                             8M KF/2.4M P.sub.2 O.sub.5                                                                  400° C./1000 atm                                                                10.53                                                  8M KF/2.4M P.sub.2 O.sub.5                                                                  600° C./2000 atm                                                                10.16                                                                              -.37                                         KTA.sup.2                                                                          4.8M KOH/12M KH.sub.2 AsO.sub.4                                                             400° C./1000 atm                                                                0.00                                              Control                                                                            4.8M KOH/12M KH.sub.2 AsO.sub.4                                                             600° C./2000 atm                                                                0.34 0.34                                              4M KF/8M KH.sub.2 AsO.sub.4                                                                 400° C./1000 atm                                                                1.50                                                   4M KF/8M KH.sub.2 AsO.sub.4                                                                 600° C./2000 atm                                                                2.69 1.19                                              4M KF/2M As.sub.2 O.sub.5                                                                   400° C./1000 atm                                                                5.72                                                   4M KF/2M As.sub.2 O.sub.5                                                                   600° C./2000 atm                                                                4.81 -0.91                                             8M KF/2.4M As.sub.2 O.sub.5                                                                 400° C./1000 atm                                                                12.39                                                  8M KF/2.4M As.sub.2 O.sub.5                                                                 600° C./2000 atm                                                                12.07                                                                              -0.37                                             12M KF/3.6M As.sub.2 O.sub.5                                                                400° C./1000 atm                                                                14.37                                                  12M KF/3.6M As.sub.2 O.sub.5                                                                600° C./2000 atm                                                                18.10                                                                              3.73                                         __________________________________________________________________________     .sup.1 KTiOPO.sub.4                                                           .sup.2 KTiOAsO.sub.4                                                          .sup.3 This represents the difference in solubility between runs of a         particular mineralizer at the two temperature and pressure conditions         illustrated.                                                             

It can be seen from Table 3 that the aqueous mineralizers known in theart (Controls containing no KF), provide very low solubility ofKTiOAsO₄.

EXAMPLE 4 Growth of KTiOPO₄

A mixture consisting of 3.5 ml of H₂ O, 1.63 g of KF, 1.29 g of P₂ O₅and about 1.8 g of fine (<0.5 mm³) KTiOPO₄ crystals was placed in a goldtube (0.375 inches in diameter and about 6 inches long) and both endswere hermetically sealed shut. The tube was placed in an internallyheated pressure vessel which was brought to a temperature of 600° C. and2000 arm. pressure. Argon was used to pressurize the vessel. Atemperature difference of about 25° C. was established between thecenter of the gold tube and its two ends by cooling the two ends of thepressure vessel. The temperature and pressure were maintained for 16hours during which time crystallization of KTiOPO₄ took place viatemperature gradient transport. The vessel was then quenched to roomtemperature and optical quality crystals of up to about 1.5×1×1 mm³ wererecovered from the gold tubes.

EXAMPLE 5 Growth of KTiOAsO₄

A mixture consisting of 3.5 ml of H₂ O, 1.63 g of KF, 2.09 g of As₂ O₅and about 1.8 g of fine (<0.5 mm³) KTiOAsO₄ crystals was placed in agold tube (0.375 inches in diameter and about 6 inches long) and bothends were hermetically sealed shut. The tube was placed in an internallyheated pressure vessel which was brought to a temperature of 600° C. and2000 atm. pressure. Argon was used to pressurize the vessel. Atemperature difference of about 25° C. was established between thecenter of the gold tube and its two ends by cooling the two ends of thepressure vessel. The temperature and pressure were maintained for 16hours during which time crystallization of KTiOAsO₄ took place viatemperature gradient transport. The vessel was then quenched to roomtemperature and optical quality crystals of up to about 1.5×1×1 mm³ wererecovered from the gold tubes.

EXAMPLE 6 Growth of KTiOAsO₄

A mixture consisting of 3.5 ml of H₂ O, 0.81 g of KF, 1.38 g of As₂ O₅and about 0.46 g of fine (<0.5 mm³) KTiOAsO₄ crystals was placed in agold tube (0.375 inches in diameter and about 6 inches long) and bothends were hermetically welded shut. The tube was placed in an internallyheated pressure vessel which was brought to a temperature of 800° C. and2000 atm. pressure. Argon was used to pressurize the vessel. Thetemperature was held constant at 800° C. for 1 hour, and then cooled ata rate of 5° C. per hour to a temperature of 600° C. After holding thetemperature at 600° C. for two more hours, the vessel was quenched toroom temperature and the gold tube was cut open to remove the growncrystals. Optical quality KTiOAsO₄ crystals of up to about 2.0×1.5×1.5mm³ were recovered from the gold tubes.

EXAMPLE 7 Growth of KTiOAsO₄

A mixture of 3.5 ml H₂ O, 1.63 g KF, 0.99 g As₂ O₅, 1.91 g KH₂ AsO₄ andabout 1.33 g of fine (<0.5 mm³) KTiOAsO₄ crystals was placed in a goldtube (0.375 inches in diameter and about 6 inches long) and both endswere hermetically welded shut. The tube was placed in an internallyheated pressure vessel and brought to 400° C. and 500 atmospherespressure. Argon was used to pressurize the vessel. The temperature washeld constant at 400° C. for 5 hours, and then cooled at a rate of 5°C/hr. to 300° C. After holding at 300° C. for 8 more hours, the vesselwas quenched to room temperature and the gold tube was cut open toremove the grown crystals. Optical quality KTiOAsO₄ crystals of up toabout 1×1×1 mm³ were obtained.

The examples serve to illustrate particular embodiments of theinvention. Other embodiments will become apparent to those skilled inthe art from a consideration of the specification or practice of theinvention disclosed herein. It is understood that modifications andvariations may be practiced without departing from the spirit and scopeof the novel concepts of this invention. It is further understood thatthe invention is not confined to the particular formulations andexamples herein illustrated, but it embraces such modified forms thereofas come within the scope of the following claims.

What is claimed is:
 1. An aqueous mineralizer suitable for use in thehydrothermal production of crystals of MTiOXO₄, wherein M is selectedfrom the group consisting of NH₄, K, Rb, TI, mixtures thereof, andmixtures of Cs therewith; and X is selected from the group consisting ofP, As and mixtures thereof; said aqueous mineralizer consistingessentially of an aqueous solution of MF and at least one ofa) MOH; b)X₂ O₅ ; c) MA₂ XO₄, wherein A is H or F; or d) M₂ AXO₄ ; wherein A is Hor F;in proportions such that the molal concentration in the mineralizeris at least about 1 molal M⁺, at least about 0.2 molal X⁺⁵, and at leastabout 1 molal F⁻ ; and wherein the amount of F⁻ present in themineralizer is selected to increase the solubility of MTiOXO₄ in themineralizer and to provide an MTiOXO₄ solubility in the mineralizer ofat least 1% by weight at the operating temperature and pressure selectedfor hydrothermal production.
 2. The aqueous mineralizer of claim 1having a composition falling within the polygon defined by points I, II,Ill, IV, V, VI, 1 in FIG.
 1. 3. The aqueous mineralizer of claim 2having a composition falling within the polygon defined by points A, B,C, D, E, F, A in FIG.
 1. 4. The aqueous mineralizer of any of claims 1-3wherein M is K and X is As.
 5. The aqueous mineralizer of any of claims1-3 wherein M is K and X is P.